The present invention relates to methods and compositions useful in treating subterranean formations, and more particularly, to consolidating relatively unconsolidated portions of subterranean formations and minimizing the flow back of unconsolidated particulate material (referred to collectively herein as “particulate migration.”) This invention also relates to modifying the stress-activated reactivity of subterranean fracture faces and other surfaces in subterranean formations.
In the production of hydrocarbons from a subterranean formation, the subterranean formation preferably should be sufficiently conductive to permit desirable fluids, such as oil and gas, to flow to a well bore that penetrates a subterranean formation. One type of treatment that may be used to increase the conductivity of a subterranean formation is hydraulic fracturing. Hydraulic fracturing operations generally involve pumping a treatment fluid (e.g., a fracturing fluid or a “pad” fluid) into a well bore that penetrates a subterranean formation at a sufficient hydraulic pressure to create or enhance one or more fractures in the subterranean formation. The fluid used in the treatment may comprise particulates, often referred to as “proppant particulates,” that are deposited in the resultant fractures. These proppant particulates are thought to prevent the fractures from fully closing upon the release of hydraulic pressure, forming conductive channels through which fluids may flow to a well bore. The term “propped fracture” as used herein refers to a fracture (naturally-occurring or otherwise) in a portion of a subterranean formation that contains at least a plurality of proppant particulates. The term “proppant pack” refers to a collection of proppant particulates within a fracture.
A type of particulate migration that may affect fluid conductivity in the formation is the flow back of unconsolidated particulate material (e.g., formation fines, proppant particulates, etc.) through the conductive channels in the subterranean formation, which can, for example, clog the conductive channels and/or damage the interior of the formation or equipment. There are several known techniques used to control particulate migration, some of which may involve the use of consolidating agents. The term “consolidating agent” as used herein includes any compound that is capable of minimizing particulate migration in a subterranean formation and/or modifying the stress-activated reactivity of subterranean fracture faces and other surfaces in subterranean formations.
One well-known technique used to control particulate migration in subterranean formations is commonly referred to as “gravel packing,” which involves the placement of a filtration bed of gravel particulates in the subterranean formation that acts as a barrier to prevent particulates from flowing into the well bore. These gravel packing operations may involve the use of consolidating agents to bind the gravel particulates together in order to form a porous matrix through which formation fluids can pass.
In some situations, a hydraulic fracturing treatment and a gravel-packing treatment may be combined into a single treatment (commonly referred to as FRACPAC™ operations). In such “frac pack” operations, the fracturing and gravel-packing treatments are combined and may generally be completed with a gravel pack screen assembly in place with the hydraulic fracturing treatment being pumped through the annular space between the casing and screen. In this situation, the hydraulic fracturing treatment ends in a screen-out condition, creating an annular gravel pack between the screen and casing. In other cases, the fracturing treatment may be performed prior to installing the screen and placing a gravel pack.
Another technique that may be used to control particulate migration involves coating proppant particulates with a consolidating agent to facilitate their consolidation within the formation and to prevent their subsequent flow-back through the conductive channels in the subterranean formation.
Another method used to control particulate migration involves consolidating unconsolidated portions of subterranean zones into relatively stable permeable masses by applying a consolidating agent to an unconsolidated portion of the subterranean formation. One example of this method is applying a resin to a portion of the zone, followed by a spacer fluid and then a catalyst. Such resin application may be problematic when, for example, an insufficient amount of spacer fluid is used between the application of the resin and the application of the external catalyst. In that case, the resin may come into contact with the external catalyst earlier in the process such as in the well bore itself rather than in the unconsolidated subterranean formation. Furthermore, there may be uncertainty as to whether there is adequate contact between the resin and the catalyst. The terms “catalyst,” “hardening agent,” and “curing agent” may be used herein interchangeably and collectively may refer to a composition that effects the hardening of a resin composition by any means or mechanism. Another example of this method involves applying a tackifying composition (aqueous or non-aqueous) to a portion of the formation in an effort to reduce the migration of particulates therein. Whereas a curable resin composition produces a hard mass, the use of a tackifying composition is thought to result in a more malleable consolidated mass.
Although consolidating agents are used frequently, they may be difficult to handle, transport and clean-up due to their inherent tendency to stick to equipment or anything else with which they may come into contact. Therefore, it would be desirable to provide compositions and methods that would, among other things, help ease the handling, transport and clean up when using consolidating agents.
One additional problem that can negatively impact conductivity and further complicate the effects of particulate migration is the tendency of mineral surfaces in a subterranean formation to undergo chemical reactions caused, at least in part, by conditions created by mechanical stresses on those minerals (e.g., fracturing of mineral surfaces, compaction of mineral particulates). These reactions are herein referred to as “stress-activated reactions” or “stress-activated reactivity.” As used herein, the term “mineral surface in a subterranean formation” and derivatives thereof refer to any surface in a subterranean formation comprised of minerals and/or the surface of a particulate. These minerals may comprise any mineral found in subterranean formations, including silicate minerals (e.g., quartz, feldspars, clay minerals), carbonaceous minerals, metal oxide minerals, and the like. The mineral surface in a subterranean formation treated in the methods of the present invention may have been formed at any time. The term “modifying the stress-activated reactivity of a mineral surface” and its derivatives as used herein refers to increasing or decreasing the tendency of a mineral surface in a subterranean formation to undergo one or more stress-activated reactions, or attaching a compound to the mineral surface that is capable of participating in one or more subsequent reactions with a second compound.
One type of reaction caused, at least in part, by conditions created by mechanical stresses on minerals may be referred to as a diagenic reaction, which also may be known as a “diageneous reaction.” As used herein, the terms “diagenic reaction,” “diagenic reactivity,” and “diagenesis” or any derivatives thereof include chemical and physical processes that move a portion of a mineral sediment and/or convert the mineral sediment into some other mineral form in the presence of water. A mineral sediment that has been so moved or converted is herein referred to as a “diagenic product.” Any mineral sediment may be susceptible to these diagenic reactions, including silicate minerals (e.g., quartz, feldspars, scale, clay minerals), carbonaceous minerals, metal oxide minerals, and the like.
Two mechanisms that diagenic reactions are thought to involve are pressure solution and precipitation processes. Where two water-wetted mineral surfaces are in contact with each other at a point under strain, the localized mineral solubility near that point is thought to increase, causing the minerals to dissolve. Minerals in solution may diffuse through the water film outside of the region where the mineral surfaces are in contact (e.g., in the pore spaces of a proppant pack), where they may precipitate out of solution. The dissolution and precipitation of minerals in the course of these reactions may reduce the conductivity of the formations by, among other things, clogging the conductive channels in the formation with mineral precipitate and/or collapsing those conductive channels by dissolving solid minerals in the surfaces of those channels.
Moreover, in the course of a fracturing treatment, new mineral surfaces may be created in the “walls” surrounding the open space of the fracture. These new walls created in the course of a fracturing treatment are herein referred to as “fracture faces.” Such fracture faces may exhibit different types and levels of reactivity, for example, stress-activated reactivity. In some instances, fracture faces may exhibit an increased tendency to undergo diagenic reactions. In other instances, fracture faces also may exhibit an increased tendency to react with substances in formation fluids and/or treatment fluids that are in contact with those fracture faces, such as water, polymers (e.g., polysaccharides, biopolymers, surfactants, etc.), and other substances commonly found in these fluids, whose molecules may become anchored to the fracture face. This reactivity may further decrease the conductivity of the formation through, inter alia, increased diagenic reactions and/or the obstruction of conductive fractures in the formation by any molecules that have become anchored to the fracture faces.